Parallax scroll event optimization for changing background images and not disrupting the fluid scroll

parallax.js

/*
    Used in conjunction with Scott Robin's backstretch script:
    https://github.com/srobbin/jquery-backstretch
*/
	
(function($) {

var justExecuted = false;

$(window).on('scroll', function() {
	// Slow down event execution with timeout set below
	if ( justExecuted ) {
		return;
	}

	// Cache all values and elements used below
	var width, 
	    backStretch,
	    isFixed,
	    $body = $('body'),
	    $menu = $('.menu'),
	    $backStretchEl = $('.backstretch img'),
	    $scrollPosition = $body.scrollTop();

	// console.log($scrollPosition); // for testing breakpoints

	// Switch became increasingly efficient here
	switch( true ) {
		case ($scrollPosition < 150):
			if ( backStretch == 1 ) return;
			$backStretchEl.attr('src', 'images/bg1.jpg'); 
			backStretch = 1;
			break;

		case ($scrollPosition < 1000):
			if ( backStretch == 2 ) return;
			$backStretchEl.attr('src', 'images/bg2.jpg'); 
			backStretch = 2;
			break;

		case ($scrollPosition < 1700):
			if ( backStretch == 3 ) return;
			$backStretchEl.attr('src', 'images/bg3.jpg'); 
			backStretch = 3;
			break;

		case ($scrollPosition < 3000):
			if ( backStretch == 4 ) return;
			$backStretchEl.attr('src', 'images/bg4.jpg'); 
			backStretch = 4;
			break;

		case ($scrollPosition < 4300):
			if ( backStretch == 5 ) return;
			$backStretchEl.attr('src', 'images/bg5.jpg'); 
			backStretch = 5;
			break;
	}

	// Reset timeout periodically (increase for better performance, but decreased accuracy)
	justExecuted = true;
	setTimeout(function() {
		justExecuted = false;
	}, 100);

});

})(jQuery);